krb5: append to LDFLAGS instead of replacing

[chs]

Summary

Sponsored by: Netflix

[ivy]

is there any way we could consolidate this in a single Makefile rather than sprinkling it all over the place?

[des]

Why is this needed? Do these libraries depend on symbols which the application is expected to define?

[chs]

This patch does not change what any of these Makefiles add to LDFLAGS. What is being changed by this patch to have these Makefile add their options to any existing contents of $LDFLAGS by using "LDFLAGS += ..." rather than discarding any existing contents of $LDFLAGS by using "LDFLAGS = ..." . I have no idea why any of these Makefiles want to add anything to LDFLAGS or if there is a better way to accomplish their goal, all I want to change is to add the "+'" in between "LDFLAGS" and "=".

These Makefiles will then match what almost all other Makefiles in base do, with the only exceptions being stuff like third-party Makefiles in src/contrib that are not used, and Makefiles in places src/stand where what is being built is not a normal application or shared library.